I finally got around to making a copy of Pentagrid's fixture, which was detailed in the thread "Re: Fixture for boring soft jaws on a lathe" posted in early May 2009.
Pentagrid's fixture consists of three pieces of spring steel 0.5" wide by 0.080" (2 mm) thick by about 6". held in grooves milled in the sides to the three soft jaws. The strips form an equilateral triangle, at least initially.
My implementation uses scale-free spring steel 0.5" wide by 0.093" thick by 6.312", long resting in milled grooves ~0.368" wide and 0.160" deep. The ends of the spring steel strips are cut square to the length of the trip, and roughly rounded in the thickness so they will bottom in the corner of the milled grooves and not hang up on tool marks on the walls.
The grooves are parallel to the chuck rotation axis, and there are two grooves per jaw, one on each side.
The 2mm (0.080") thickness isn't common in the US, so I used 0.094" nominal (2.4 mm).
The spring steel is # 03342185 from MSC: .
It all worked quite well, but it took a LOT of force to tighten the chuck. I had to use a cheater pipe on the chuck key. The spring steel strips now have the desired bend in the center, so they now won't tend to bow inward. I suppose there is no harm in using the cheater if not done too often, and the next time may be easier now that the springs are broken in A thinner spring may also be adequate, and require less force..
Joe Gwinn